This invention relates to an instrument for measuring terrain conductivity.
It is well known that terrain conductivity measurements provide useful information for purposes of geological mapping, and techniques have been developed for that purpose that employ non-ground contacting electromagnetic devices that are either airborne or for use on the ground. An example of a widely used electromagnetic device for measuring ground conductivity is disclosed in U.S. Pat. No. 4,070,612 issued Jan. 24, 1978, to McNeill et al. and assigned to the assignee of the present invention.
With reference to FIG. 1, a commonly used technique for measuring ground conductivity involves energizing a transmitting coil Tx that is located on or just above the surface of the earth with an alternating current at an audio frequency into a transmitting coil Tx. The time varying magnetic field from this alternating current induces very small eddy currents in the earth, which in turn generate a secondary magnetic field that is sensed by a receiver coil Rx that is located a short distance s away from the transmitting coil Tx. In general, the secondary magnetic field is a complicated function of the intercoil spacing s, the operating frequency f, and the ground conductivity "sgr", with the ratio of secondary to primary magnetic field represented by:                                           Hs            Hp                    =                                    2                                                (                                      γ                    ⁢                                          xe2x80x83                                        ⁢                    s                                    )                                2                                      ⁢                          {                              9                -                                                      [                                          9                      +                                              9                        ⁢                                                  xe2x80x83                                                ⁢                        γ                        ⁢                                                  xe2x80x83                                                ⁢                        s                                            +                                              4                        ⁢                                                                              (                                                          γ                              ⁢                                                              xe2x80x83                                                            ⁢                              s                                                        )                                                    2                                                                    +                                                                        (                                                      γ                            ⁢                                                          xe2x80x83                                                        ⁢                            s                                                    )                                                3                                                              ]                                    ⁢                                      ⅇ                                                                  -                        γ                                            ⁢                                              xe2x80x83                                            ⁢                      s                                                                                  }                                      ,                            (                  Equ          .                      xe2x80x83                    ⁢          1                )            
Where:
Hs=secondary magnetic field at the receiver coil
Hp=primary magnetic field at the receiver coil
xcex3={square root over (ixcfx89xcexc0"sgr")}
xcfx892Πf
f=frequency
xcexc0=permeability of free space
"sgr"=ground conductivity (Siemen/meter)
s=intercoil spacing (meter)
i={square root over (xe2x88x921)}
As explained in J. D. McNeill, xe2x80x9cGeonics Limited Technical Note TN-6xe2x80x94Electronic Terrain Conductivity Measurement at Low Induction Numbersxe2x80x9d, Geonics Limited, 1980, under certain constraints the ratio of secondary to primary magnetic fields is a relatively simple function of the above variables, namely:                                           Hs            Hp                    ≅                                    i              ⁢                              xe2x80x83                            ⁢                              ωμ                0                            ⁢              σ              ⁢                              xe2x80x83                            ⁢                              s                2                                      4                          ,                            (                  Equ          .                      xe2x80x83                    ⁢          2                )            
Given the ratio of secondary to primary magnetic fields, the apparent conductivity "sgr"a can be calculated as:                                           σ            a                    =                                    4                              ω                ⁢                                  xe2x80x83                                ⁢                                  μ                  0                                ⁢                                  s                  2                                                      ⁢                          (                              Hs                Hp                            )                                      ,                            (                  Equ          .                      xe2x80x83                    ⁢          3                )            
The linear relationship between the ratio of secondary to primary magnetic fields and the conductivity of the terrain being surveyed is typically maintained as long as the coil separation is less than about one tenth of skin depth. In most practical cases, the secondary magnetic field Hs is a very small fraction of the primary magnetic field Hp. For example, with a coil separation of s=2 m, frequency of operation f=20 kHz and ground conductivity of 1 mS/m, the ratio of Hs/Hp=1.58xc3x9710xe2x88x924. Since most often secondary magnetic field Hs is measured in the presence of a primary magnetic field Hp that is many of orders of magnitude larger, great care must be taken to maintain stability of the measuring system to accurately measure the secondary magnetic field (and indirectly ground conductivity), especially in areas where terrain conductivity is low.
In order to maintain stability in a cost effective manner in prior ground conductivity measuring systems, the number of coils has typically been limited to one transmitting coil and at most two receiver coils, with instruments having only one receiver coil being more common. As it is sometimes desirable to take multiple measurements of the same terrain location with different coil orientations and/or spacings, the lack of a plural coil pairs in previous EM based conductivity measuring instruments has led to decreased efficiency in performing measurements as the measurement process has to be repeated for each different coil orientation or spacing.
Furthermore, as ambient temperature variation tends to affect the mutual coupling and interaction between the ground and the receiver and transmitter coils, the stability of previous systems has been adversely affected by temperature changes. Attempts have been made to use analog circuitry, including temperature sensitive resistors, in ground conductivity measuring systems to compensate for the effect of temperature changes on such systems. An example of an EM measuring device that employs an analog temperature compensation system is the Geonics EM31 (trademark), that is available from Geonics Limited of Mississauga, Ontario, Canada. Although useful in many applications, analog temperature compensation techniques tend to have limited success in correcting for temperature drift that does not vary in a substantially linear fashion with temperature change.
Thus, there is a need for a ground conductivity measuring device that uses multiple transmitters and receivers in a configuration that permits system stability to be maintained, and for a ground conductivity measuring device that operates with stability through a wide range of ambient temperatures.
According to one aspect of the invention, there is provided a conductivity meter for measuring conductivity of terrain, including a first transmitter coil, a signal generator connected to the first transmitter coil to supply a time-varying current thereto for inducing eddy currents in the terrain, a first receiver coil horizontally spaced from the first transmitter coil, a temperature sensing device for measuring temperature and a signal processor. The signal processor includes a memory storing a plurality of temperature dependent correction values, and is configured to isolate from a signal received by the first receiver coil a secondary signal representative of a secondary magnetic field generated in the terrain by the eddy currents, and determine an apparent terrain conductivity based on the isolated secondary signal and a selected temperature dependent correction value selected from the stored temperature dependent correction values according to a measured temperature received from the temperature sensing device. Preferably, the temperature sensing device includes a sensor positioned to measure the first receiver coil temperature.
According to the invention, there is also provided a method of measuring terrain conductivity that includes: (a) generating an AC signal and applying it to a transmitter coil positioned over the terrain whose conductivity is being measured; (b) receiving signals from the transmitter coil by means of a receiver coil horizontally spaced from the transmitter coil; (c) isolating from the received signals a signal representative of a secondary magnetic field generated in the terrain by eddy currents resulting from a primary magnetic field generated by the transmitter coil; (d) measuring a temperature of the receiver coil and selecting based thereon a correction value from a plurality of pre-determined temperature dependent correction values; and (e) calculating an apparent conductivity according to the selected correction value and a magnitude of the isolated signal.
According to another aspect of the invention, there is provided a conductivity meter for measuring conductivity of terrain, including a transmitter coil, a signal generator connected to the first transmitter coil to supply a time-varying current thereto for inducing currents in the terrain, a plurality of receiver coils horizontally spaced from the transmitter coil at different distances and a signal processor configured to isolate, for each receiver coil, an associated secondary signal representative of a secondary magnetic field generated in the terrain by the induced currents and received by the receiver coil, and determine, for each isolated secondary signal an apparent conductivity based on the isolated secondary signal.
According to still a further aspect of the invention, there is provided a conductivity meter for measuring conductivity of terrain, including a first transmitter coil and a second transmitter coil located proximate to each other and having perpendicular dipoles, a signal generator connected to the first transmitter coil and the second transmitter coil to supply a time-varying current thereto for generating perpendicular primary magnetic fields inducing currents in the terrain, a first receiver coil coplanar with and horizontally spaced from the first transmitter coil, a second receiver coil coplanar with and horizontally spaced from the second transmitter coil in the same direction and substantially the same distance that the first receiver coil is spaced from the first transmitter coil, the first transmitter and first receiver coils having parallel dipoles and the second transmitter and second receiver coils having parallel dipoles, and a signal processor. The signal processor is configured to isolate, for the first receiver coil, a secondary signal representative of a secondary magnetic field generated in the terrain by the current induced therein by a primary field generated by the first transmitter coil, and to isolate for the second receiver coil, a secondary signal representative of a secondary magnetic field generated in the terrain by the current induced therein by a primary field generated by the second transmitter coil, the signal processor being configured to determine, for each isolated secondary signal an apparent conductivity based on the isolated secondary signal. Preferably, the first transmitter and second transmitter coils are wound in perpendicular plans about a common coil former and the first receiver and second receiver coils are wound in perpendicular plans about a further common coil former.